The pollutants contained in the exhaust gases of an engine and resulting from the combustion of a fuel mixture are mainly unburnt hydrocarbons (HC), carbon monoxide (CO) and nitrogen oxides (NO and NO2), more commonly referred to as NOx.
In order to comply with environmental standards and to meet the severization of these standards, such as the standards known as EURO V or EURO VI, these pollutants have to be treated prior to discharging the exhaust gases into the atmosphere.
As it is generally known, pollutant post-treatment devices are installed on the exhaust line of vehicles. Thus, HC and CO treatment is achieved through passage of these exhaust gases through a triple-effect oxidation catalyst known as three-way catalyst, by means of which the HC and the CO are oxidized. Furthermore, the NOx are reduced under particular engine running conditions such as operation with a stoichiometric fuel mixture, through passage through this catalyst.
In order to be able to treat the NOx, in cases where the gases are globally oxidizing, these gases also flow through another catalyst, referred to as SCR (Selective Catalytic Reduction) catalyst, which allows to selectively reduce the NOx to nitrogen through the action of a reducing agent. This agent can be ammonia or a compound generating ammonia through decomposition, such as urea, which is generally injected upstream from the SCR catalyst. This ammonia mixes with the exhaust gases, then it reacts with the NOx of the exhaust gases on the SCR catalyst according to several possible chemical reactions.
This agent can also be a hydrocarbon, hydrogen, carbon monoxide, . . . .
As described in particular in document EP-0,758,714, such a plant comprises the exhaust line of an internal-combustion engine on which are installed a three-way oxidation catalyst, means for injecting a reducing agent and a selective catalytic reduction catalyst.
The problem that arises with such a plant is that the three-way catalyst operates only when it has reached its light-off temperature, close to 200° C., with a stoichiometric fuel mixture, and the SCR catalyst is operational with a lean fuel mixture with an operating temperature ranging between 180° C. and 550° C.
This is why there are two exhaust gas circulation channels, as illustrated in the aforementioned document, one comprising the three-way catalyst and the SCR catalyst, and the other circulation channel opening directly onto the inlet of the three-way catalyst.
Besides the fact that this plant is of complicated design and voluminous with two circulation channels, it also requires throttle means associated with control strategies to control circulation of the exhaust gases through one of the channels or the other.
Furthermore, when the channel comprising the SCR catalyst is supplied with exhaust gases, very hot exhaust gases flow through the catalyst and therefore generate a thermal shock. Such a thermal shock can damage it, notably if it is repetitive and more particularly when the SCR catalyst is at ambient temperature.
The present invention aims to overcome the aforementioned drawbacks by means of an exhaust gas treatment plant of simple design and reduced cost.